US scientists have given details of how they managed to get the unfertilised eggs of monkeys to start dividing like embryos - a process known as parthenogenesis - and then "harvest" them for special cells.

I don't think the fact that a parthenogenetic embryo is not viable solves the ethical problems for those who object to using human embryos for stem cells

Dr Donald Bruce, bioethicist

Parthenogenesis is well documented in insects and lizards but is not normal in higher animals. The researchers, from Advanced Cell Technology (ACT) and the Wake Forest University School of Medicine, are trying to develop the technique in humans.

They believe parthenogenic embryos could provide a useful source of stem cells, the "master" cells with which scientists hope one day to treat ailing hearts and diseased brains.

Researchers think that because parthenotes are incapable of developing into a foetus, their use may bypass some of the ethical objections that have dogged current experiments on normal embryos.

"Parthenogenesis offers an important new therapeutic strategy for a host of medical conditions," said Wake Forest scientist Dr Kent Vrana.

Brain chemical

Parthenotes have been created from mice eggs - and even monkey eggs before. But this new research, announced at a conference in December and now formally published in the journal Science, goes a few steps further.

Monkey parthenotes: A pointer to "ethical" cloning?

It shows that 77 monkey eggs were exposed to chemicals designed to make them think they had been fertilised by sperm. Twenty-eight of the eggs started dividing like embryos, with four continuing the development up to the blastocyst stage, where researchers can extract the stem cells that are the "parents" of all the tissues in the body.

The research team used more chemical triggers to direct their monkey stem cells into becoming muscle and fat cells and even beating heart cells. But, said Professor Vrana and co-researcher Professor Kathleen Grant, the most remarkable differentiation was the development of neurons that produced the important brain chemical lacking in Parkinson's sufferers, dopamine.

"This is a specialised population of neurons whose efficient generation from primate embryonic stem cells had not been reported previously," they said in a statement.

The monkey stem cells were also injected into living mice to further demonstrate that they could develop into useful cell types.

Still to prove

ACT was the company which controversially claimed last year to have produced the world's first human embryo clones. As part of that research project, it also attempted to produce human parthenotes - but none of its embryos developed to blastocyst stage.

The potential clinical applications include treatment of diseases where specific cell types have become dysfunctional

Dr Michael West, ACT

However, this latest work would appear to indicate that such a stage could soon be reached in humans - although Davor Solter, of the Max Planck Institute for Immunobiology in Germany, remains sceptical.

"Every single mammal has its own quirks," he told Science. "If you want to figure out how to make parthenotes in humans, you have to make them in humans."

Dr Michael West, who heads Advanced Cell Technology, is confident the goal can be achieved and the human blastocysts mined for their valuable stem cells.

"The potential clinical applications include treatment of diseases where specific cell types have become dysfunctional," he said. "These diseases include a broad array of medical problems, such as Parkinson's and Huntington's diseases, heart disease and diabetes."

'Holistic' view

But such applications would most probably be limited only to women of reproductive age. Men do not produce eggs and to transplant into them tissues derived from parthenotes containing a female genetic "signature" would invite rejection.

Chemical triggers directed the development of stem cells

And the leading bioethicist Dr Donald Bruce doubted whether parthenogenesis would satisfy those who objected to embryology research anyway.

"If these primate embryos created by parthenogenesis seem to have the properties of embryos - to the point that stem cells can be derived and differentiated into various body cell types - then what we have is an embryo," the director of the Church of Scotland's Society, Religion and Technology Project said.

To suggest that such embryos were more acceptable because they could not develop into a foetus was "a 'technical fix' argument which would not satisfy a more holistic view, which would see this as no different in status from a non-viable cloned embryo or a non-viable IVF embryo".